Diffusion-controlled products
In these systems, there is a water-insoluble polymer which controls the flow of water and the subsequent egress of dissolved drug from the dosage form. Both diffusional and dissolution processes are involved. In `reservoir' devices, a core of drug is coated with the polymer and, in `matrix' systems, the drug is dispensed throughout the matrix. Cellulose derivatives are commonly used in the reservoir types, while the matrix material may be plastics, e.g. methylacrylate-methyl methacrylate, polyvinyl chloride, hydrophilic polymers such as cellulose derivatives or fatty compounds including carnauba wax. Examples of this type of formulation include Plendil ER, Agon SR, Kapanol and Slow-K.
Dissolution-controlled products
In these products, the rate of dissolution of the drug (and thereby availability for absorption) is controlled by slowly soluble polymers or by micro encapsulation. Once the coating is dissolved, the drug becomes available for dissolution. By varying the thicknesses of the coat and its composition, the rate of drug release can be controlled. Some preparations contain a fraction of the total dose as an immediate-release component to provide a pulse dose soon after administration. The pellet dosage forms of diffusion- or dissolution-controlled products can be encapsulated or prepared as a tablet. These products should not be chewed as the coating may be damaged. One of the advantages of encapsulated pelleted products is that the onset of absorption is less sensitive to stomach emptying. The entrance of the pellets into the small intestine (where the majority of drug absorption occurs) is usually more uniform than with non-disintegrating extended-release tablet formulations. An example of this type of product is Fefol.
Erosion products
The release of drug from these products is controlled by the erosion rate of a carrier matrix. The rate of release is determined by the rate of erosion. An example of this formulation is Sinemet CR. With this product, some patients may experience a later onset of effect after the morning dose, compared to conventional levodopa tablets, because of the delayed release of the drug.
Osmotic pump systems
The rate of release of drug in these products is determined by the constant inflow of water across a semipermeable membrane into a reservoir which contains an osmotic agent. The drug is either mixed with the agent or is located in a reservoir. The dosage form contains a small hole from which dissolved drug is pumped at a rate determined by the rate of entrance of water due to osmotic pressure. The rate of release is constant and can be controlled within tight limits yielding relatively constant blood concentrations. The advantage of this type of product is that the constant release is unaltered by the environment of the gastrointestinal tract and relies simply on the passage of water into the dosage form. The rate of release can be modified by altering the osmotic agent and the size of the hole. An example of this type of product is Adalat Oros.
Ion exchange resins
Some drugs can be bound to ion exchange resins and, when ingested, the release of drug is determined by the ionic environment within the gastrointestinal tract. Examples of this type of product are Duromine containing the basic drug phentermine complexed onto an anionic resin, and MS Contin suspension which uses a polystyrene sulphonate resin.